The present invention relates to a time base, i.e. a device comprising a resonator and an integrated electronic circuit for driving the resonator into oscillation and for producing, in response to this oscillation, a signal having a determined frequency.
Time bases, or frequency standards, are required in a large variety of electronic devices, ranging from wristwatches and other timepieces to complex telecommunication devices. Such time bases are typically formed by an oscillator including a quartz resonator and an electronic circuit for driving the resonator into oscillation. An additional division chain may be used to divide the frequency of the signal produced by the oscillator in order to obtain a lower frequency. Other parts of the circuit may serve to adjust the frequency, for example by adjusting the division ratio of the division chain. The components of the electronic circuit are advantageously integrated onto a single semiconductor substrate in CMOS technology. Other functions, not directly related to the frequency processing, may be integrated onto the same substrate.
Advantages of quartz resonators are their high quality factor Q leading to good frequency stability and low power consumption as well as their good temperature stability. A disadvantage of typical time bases using quartz resonators however resides in the fact that two components, namely the quartz resonator and the integrated electronic circuit, are required in order to provide a high-precision frequency. A discrete quartz resonator requires board space which is scarce in many cases. For instance, a standard quartz resonator for wristwatch applications requires space of the order of 2xc3x972xc3x976 mm3. Moreover, additional costs are caused by the assembly and connection of the two components. Yet, space and assembly costs are major issues, especially in the growing field of portable electronic devices.
It is thus a principal object of the present invention to provide a solution to the above-mentioned problems by providing a time base comprising an integrated resonator.
Another object of the present invention is to provide a time base that may be fully integrated on a single substrate, that is suitable for mass production and that is compatible with CMOS technology.
Still another object of the present invention is to provide a time base comprising a resonator having an improved quality factor Q and thereby a greater frequency stability and low power consumption.
Yet another object of the present invention is to provide such a time base which is low-priced and requires only a very small surface area on a semiconductor chip.
Accordingly, there is provided a time base comprising a resonator and an integrated electronic circuit for driving said resonator into oscillation and for producing, in response to said oscillation, a signal having a determined frequency, characterised in that said resonator is an integrated micromechanical tuning fork resonator supported above a substrate and adapted to oscillate, according to a first oscillation mode, in a plane substantially parallel to said substrate, said tuning fork resonator comprising a base member extending substantially perpendicularly from said substrate, a free-standing oscillating structure connected to said base member and including at least a first pair of substantially parallel fork tines disposed in said plane, and an electrode structure disposed adjacent to said fork tines and connected to said integrated electronic circuit.
An advantage of the time base according to the present invention lies in the fact that the micromechanical tuning fork resonator exhibits a high quality factor Q.
Quality factors as high as 50""000 have been measured which is of the same order as those obtained using conventional quartz resonators. The quality factor Q is determined by air friction and by intrinsic losses in the vibrating resonator material. Air friction can be neglected if the resonator is operated under vacuum conditions. Intrinsic losses depend on the material as well as on the design of the resonator. Resonators made of crystalline materials, like quartz or silicon, are known to be capable of high-Q oscillation. In addition, the clamping, i.e. the mechanical support of the resonator part, strongly influences the dynamic behaviour. According to the present invention, the tuning fork resonator is designed and driven in such a way that the centre of gravity of the entire structure remains motionless during oscillation and that the bending moments of the fork tines can be compensated in a relatively small region of the base member. Different design features favouring a high quality factor Q are the object of the dependent claims and will be described hereinafter in detail.
In addition, for a given resonant frequency, the surface area required on the substrate to form the tuning fork resonator is small in comparison with other resonators. For instance, a tuning fork resonator according to the present invention designed for a frequency of 32 kHz requires a chip area of approximately 0.2 mm2 which is smaller than the chip area required by the silicon ring resonator described in pending international application No. PCT/CH 00100583 filed on Nov. 1, 2000 by the same Applicant.
According to one aspect of the invention, the electronic circuit is advantageously integrated on the substrate together with the micromechanical tuning fork resonator, thereby leading to a low-priced time base. A lower price is also obtained by vacuum-sealing of the resonator at the wafer-level in a batch-process using wafer-bonding technology.
Tuning fork structures have been proposed as resonating structures for different types of sensor applications, such as acceleration, rotation or strain sensors. These sensor structures are, however, not optimised according to the same guidelines as in the present invention where a high quality factor is a primary goal in order to obtain a highly precise time base.
U.S. Pat. No. 5,747,691 to Yoshino et al. for instance describes a tuning fork-shaped vibratory element made of a single crystalline silicon substrate. The tines of the tuning fork have thin and thick regions in order to allow a bending of the arm in a direction perpendicular to the oscillation when an external force is applied. The resonating element has been optimised with respect to the sensor application.
GB Patent No. 2,300,047 to Fitzpatrick et al. describes an assembly of tuning fork sensors in order to provide a three-dimensional motion detection.
Yet other documents, e.g. WO 91103716 to Jensen et al., GB 2,162,314 to Greenwood et al., U.S. Pat. No. 4,912,990 to Norling et al., or the article of Beeby et al. in the Journal of Microelectromechanical Systems, Vol. 9, No. 1 (2000), pp. 104 ff., describe micromachined silicon resonant strain gauges in the form of a double-ended tuning fork.
None of the above-cited documents however indicates or suggests using such a type of tuning fork resonator in an oscillator circuit to act as a frequency standard or time base. Moreover, a number of design features of the tuning fork resonators disclosed in these documents render them less suitable for horological applications where frequency stability and low power consumption are essential.
Anisotropically etched oscillating tuning fork structures have been described previously by the present inventors. Anisotropic etching of the structure inevitably yields, however, a different length of the fork tines, which results, in turn, in a low quality factor for such a tuning fork resonator. The fabrication method and disadvantages of such anisotropically etched tuning fork resonators are discussed in greater details in the article by M. Giousouf et al., published in Proc. of Eurosensors XII, Vol. 1 (1998), pp. 381-384, or the article by M. Giousouf et al., published in Sensors and Actuators 76 (1999), pp. 416-424, both entitled xe2x80x9cStructuring of Convex Corners using a Reoxidation Process-Application to a Tuning Fork Resonator made from (110)-Siliconxe2x80x9d. Those skilled in the art will easily understand that this type of resonating structures is particularly unsuited to form a high-precision time base. Q factors of around 1000 have been measured in vacuum on such structures, which is by far too low for an application as a frequency standard.
Tuning fork resonators according to the present invention are optimised to yield oscillation with a high quality factor Q, low power consumption and to require a very small surface area on the chip. The resonator can be driven at voltages as low as 1 V which makes the use of batteries as power source in portable electronic devices possible. Furthermore, design features are presented which facilitate the mass production of such resonators due to an enhanced tolerance with respect to technological process variations.
According to another aspect of the present invention, a temperature measuring circuit may be integrated on the substrate in order to compensate for the effect of temperature on the frequency of the signal produced by the time base. Such compensation of the resonator""s temperature dependency may easily be effected since the tuning fork resonator of the present invention has the advantage of exhibiting substantially linear temperature characteristics.
According to still another aspect of the present invention, a second micromechanical tuning fork resonator may be formed on the same substrate in order to allow temperature compensation. According to another aspect of the invention, temperature compensation is also achieved by using a single micromechanical tuning fork resonator which is operated simultaneously with two oscillation modes having different resonant frequencies.
Other aspects, features and advantages of the present invention will be apparent upon reading the following detailed description of non-limiting examples and embodiments made with reference to the accompanying drawings.
FIG. 1 is a top view illustrating schematically a time base according to the present invention comprising a micromechanical tuning fork resonator and an integrated electronic circuit;
FIG. 2 is a top view of a first embodiment of the time base of FIG. 1 comprising a micromechanical tuning fork resonator realised by means of silicon surface micromachining techniques;
FIGS. 2a and 2b are two cross-sectional views of the embodiment of FIG. 2 taken along lines A-Axe2x80x2 and B-Bxe2x80x2 respectively;
FIG. 3 is a top view of a second embodiment of the time base of FIG. 1 comprising a micromechanical tuning fork resonator which is fabricated using a substrate with a buried oxide layer, such as a silicon-on-insulator (SOI) substrate;
FIGS. 3a and 3b are two cross-sectional views of the embodiment of FIG. 3 taken along lines A-Axe2x80x2 and B-Bxe2x80x2 respectively;
FIG. 4 is a top view of a third embodiment of the time base of FIG. 1 comprising a micromechanical tuning fork resonator which is fabricated using a substrate with a buried oxide layer and by etching the backside of the substrate in order to release the fork tines;
FIGS. 4a and 4b are two cross-sectional views of the embodiment of FIG. 4 taken along lines A-Axe2x80x2 and B-Bxe2x80x2 respectively;
FIGS. 5a and 5b respectively show a first and a second in-plane oscillation mode where the fork tines oscillate in an asymmetric or xe2x80x9cin-phasexe2x80x9d manner and in a symmetric or xe2x80x9canti-phasexe2x80x9d manner respectively;
FIGS. 6a and 6b are two partial top views illustrating examples of designs of comb-shaped electrode structures;
FIG. 7 is a diagram illustrating the relationship between the voltage applied on the electrodes and the resulting electrostatic force on the fork tines;
FIGS. 8a to 8c show partial top views of three different designs intended to prevent the tuning fork tines from sticking on the electrode structures;
FIG. 9 shows a top view illustrating an improvement of the first embodiment shown in FIG. 2;
FIG. 9a is a cross-sectional view of the embodiment of FIG. 9 taken along line A-Axe2x80x2;
FIG. 10 shows part of the fork tines of the resonator with openings therein;
FIG. 11 shows two tuning fork resonators sharing the same base member and designed to exhibit two different resonant frequencies;
FIG. 12 is a top view illustrating a second mode of oscillation where the fork tines perform a vertical oscillation in opposite directions perpendicularly to the substrate plane; and
FIGS. 12a and 12b are two cross-sectional views of the illustration of FIG. 12 taken along lines A-Axe2x80x2 and B-Bxe2x80x2 respectively.
Dimensions in the drawings are not to scale.